The invention relates generally to molds for forming complexly curved products and more specifically to a mold comprising a plurality of individually formed segments which are secured to a lattice or grid type support.
The forming of a plastic, planar material such as glass into a final product such as an automobile light has been accomplished in many ways. The various processes can generally be characterized by two steps common to most: first, heating the material to a temperature sufficient to achieve a appropriate plasticity and second, conforming the material through various means such as gravity, pressure, vacuum application, and the like to a form which represents the desired final shape of the product.
In the production of lights for automobiles, this process and the apparatus utilized therein has undergone constant evolutionary change. As in many other disciplines, this change has been brought about cooperatively by ever increasing demands for complex shapes by the automotive companies and increased sophistication and capabilities of the glass forming equipment.
A significant forming technique of automobile lights and similar products is gravity forming wherein the planar product is heated to a temperature sufficient to plasticize the material, the material is placed over a form and the weight of the material itself conforms the product to the form. One of the significant disadvantages of this process is the dimensional accuracy of the finished product. Since the glass or other plastic material may not repeatedly uniformly contact the form due to variations in coefficient of friction and other process variables.
In the automotive field, increased complexity of shapes and reduced tolerances developed contemporaneously with the application of cold forming techniques. Here, heated glass is pressed between male and female mold sections to achieve a desired shape. The glass lights are suspended and gripped by tongs which move them into and out of the forming apparatus. Unfortunately, such tongs leave small marks on or adjacent the periphery of the light. As new techniques are developed for securing lights within vehicle bodies and the percent of a glass light exposed to view continues to increase, even peripheral tong marks have become unacceptable.
Another recent approach has been to utilize a fluid (air) bed to support the glass during forming as disclosed in U.S. Pat. No. 3,477,839. Here, a support bed comprising a plurality of vertically moveable parallel strips, includes openings through which heated air passes thereby softening and forming the glass. U.S. Pat. Nos. 4,483,702 and 4,522,641 also disclose glass molds. The former teaches a vacuum chamber having different levels of vacuum and a perforated face comprised of strips. The latter presents a mold having a flexible adjustable lattice type forming surface.
Various new processes which may be characterized as hot forming are now in use. One is described in U.S. Pat. No. 4,282,026. This patent discloses a furnace in which a light or similar glass shape is pressed by a peripheral ring into contact with a vacuum forming mold having a face which defines the desired shape of the product. The operation occurs at an elevated temperature which maintains the plasticity of the material. The vacuum and the peripheral ring ensure intimate contact between the material and the mold face. These features result in exceptional dimensional accuracy and minimum optical distorting.
The mold, since it is exposed to temperatures in the range of from 1300.degree. F. to 1400.degree. F., is preferably fabricated of a ceramic material. Not only are such molds massive by any production standard and therefore expensive, but they are prone to cracking and fracture. This may occur either as a result of normal production deterioration but also from accidental mishandling during transportation, setup, and teardown. Attempts to fabricate such a mold from other less costly and more durable materials have, until the present time, failed. Such attempts have centered upon the fabrication of a mold from a material such as steel. While the mold can be machined from a solid block of material, this approach is time consuming and therefore costly. One alternative is to conform a sheet of metal to the curvature of a pattern. As those familiar with sheet forming will readily appreciate, such materials generally cannot be shaped to conform to more than one curve or radius at a given location. That is, having formed the material about one axis, it is difficult to smoothly form the material into a coextensive curve about, for example, a perpendicular axis. Attempts to do so, even if marginally successful, will result in certain areas of material which are wrinkled and generally unacceptably deformed.
As light configurations become more complex, having, for example, adjacent concave and convex surfaces about parallel axes disposed between a convex surface about a perpendicular axis, the only viable molds have been those fabricated of ceramic material. Thus it is apparent that improvements in the art of construction and assembly of complexly curved forming molds are both possible and desirable.